Oxygen Transfer Rates, SOTR & Pumping Rates of Aeration Systems

How
much oxygen is your aerator really going to deliver and what is the pumping
rate. Aeration
systems make claims about the oxygen transfer rate and how many pounds
or kilograms of oxygen are added per hour or per kilowatt. This can
be a very effective way to compare the efficiency of an aeration system
based on the biological oxygen demand of the system.

A small pond with
trout or bass will have a different BOD (biological oxygen demand)
compared to a wastewater treatment system. In some cases a bottom
mounted diffuser aeration system can be the most efficient choice
but in other applications a large surface paddlewheel aerator might
be required to meet requirements.

Standard Oxygen Transfer Rates
Most commercial-grade aerators and diffuser systems have been laboratory
tested to determine their SOTR (Standard Oxygen Transfer Rate) but
the resulting SOTR ratings are based on fairly narrow criteria usually
when test water has zero dissolved oxygen and at a defined temperature.

In real world applications the advertised SOTR can almost never be
expected because the actual rate of oxygen transfer is temperature
and existing D.O. (Dissolved Oxygen) concentrations. Any aerator or
diffuser will only provide its measured (advertised maximum) oxygen
transfer rate when the dissolved oxygen levels in the water are at
zero or close to zero.

This chart will help estimate the actual oxygen transfer an aerator
will give when there is already oxygen in the water.

For example:
If the water temperature is 59°F (15°C) and the dissolved oxygen level
is 3 Parts Per Million (PPM) or 3 mg/l, an aerator system or diffuser
that is rated at 3 pounds of oxygen per hour will really only provide
64% of the advertised SOTR or around 1.92 pounds of oxygen per hour
(3 lbs x 64 percent = 1.92 lbs).

If the pond temperature is 77°F (25°C) and the dissolved oxygen level
is 5 Parts Per Million (PPM) or 5 mg/l, an aerator system or diffuser
that is rated at 3 pounds of oxygen per hour will really only provide
35% of the advertised SOTR or around 1.05 pounds of oxygen per hour
(3 lbs x 35 percent = 1.05 lbs).

Percentage
of advertised oxygen transfer actually transferred to a pond
based on water temperature and the dissolved oxygen level
prior to adding the aeration device.

Existing
Dissolved Oxygen Level
in Water Being Aerated (mg/L or PPM)

50°F
(10°C)

59°F
(15°C)

68°F
(20°C)

77°F
(25°C)

86°F
(30°C)

0

89%

90%

91%

92%

96%

1

82%

82%

82%

82%

82%

2

75%

73%

72%

72%

67%

3

67%

64%

62%

58%

56%

4

58%

55%

51%

46%

44%

5

52%

46%

41%

35%

31%

6

41%

36%

30%

24%

17%

7

34%

27%

19%

10%

3%

8

25%

17%

8%

2%

0

9

17%

8%

0

0

0

10

9%

0

0

0

0

If a pond temperature
is 68°F (20°C) and the dissolved oxygen level is 7 Parts Per Million
(PPM) or 7 mg/l, an aerator system or diffuser that is rated at 3
pounds of oxygen per hour will really only provide 19% of the advertised
SOTR or around 0.57 pounds of oxygen per hour (3 lbs x 19 percent
= 0.57 lbs).

Low oxygen concentrations can be damaging to a variety of critical
life stages of aquatic animals, including larval invertebrates, and
fish eggs and fry.

Like temperature, organisms differ in their tolerance to low dissolved
oxygen levels.

The ideal dissolved oxygen level for fish is between 7 and 9 milligrams
per liter (mg/L); most fish cannot survive at levels below 3 mg/L
of dissolved oxygen.

Some published guidelines suggest dissolved oxygen concentrations
must not decline below 5 mg/L and should not average less than 6.5
mg/L over a seven-day period. However, the guidelines also require
that dissolved oxygen concentrations remain above 9.5 mg/L in areas
where early life stages of aquatic biota, particularly fish, are present.

Anoxic (depleted oxygen) conditions can result in fishkills, which
is particularly common during harsh winters with extended ice-cover.

When fish are not a concern such as a wastewater treatment facility
or leachate pond, where the main concern is to ensure the BOD is adequately
addressed by the chosen aeration system, different criteria will apply
and take a priority role in importance when deciding if bottom diffusers,
surface aerators or some sort of jet or aspiration aerator or paddlewheel
type aerator will make the most sense.

Diffused
Air Aerators Systems
These aerators are also called lake-bed aerators or bottom-mount diffusers
and basically use a shore-mounted air blower or compressor type air
pump to push air into multiple diffusers placed at the bottom of a
pond, lake or wastewater tank.

Diffused bottom
aeration is probably the most common form of aeration in ponds and
lakes and can also be effective in wastewater basins and holding tanks.
Air is forced through a diffuser system which breaks up the airflow
into bubbles. Depending on the matrix or make-up of the diffuser the
bubbles will be fine bubbles less than 0.5 millimeters in diameter
or more coarse bubbles above 2 millimeters

Some of the more popular aeration systems in North America harness
the efficiency of the disc diffusers as part of their bottom mount,
or lake-bed, diffuser system. Diffuser discs come in various sizes;
the most common sizes are the 9 inch diffuser disc and the 12 inch
diffuser disc. These diffuser discs can be arranged on proprietary
weighted bases according to manufacturer or individually weighted
or affixed via lengths of weighted airline to mainline header systems
with valves tapped-off a main header. Diffuser discs have an integrated
check-valve system to prevent backflow of liquids into feeder lines.

Fine Bubble diffuser
discs membranes are made of EPDM (ethylene propylene diene monomer), silicone
or PTEE (PolyTetraFluoro Etyhylene) layered combinations. Lake and pond
diffuser assemblies typically use EPDM diffuser discs as they are generally
used in fairly benign, low-organic situations in fairly clean waters so
they are able to resist fouling quite well. Wastewater systems, leachate
ponds, compost reduction basins or storm water retention ponds often have
higher levels of organics and dirty effluent which can cause issues with
these diffusers and a more specialized diffuser is needed.

Establishing a SOTR (Standard Oxygen Transfer Rating) rating for a
diffuser device or to pinpoint a precise SAE (Standard Aeration Efficiency)
for a bottom mounted or lake-bed diffuser aeration system is not easy.

The SAE
(Standard Aeration Efficiency) of air diffusers is generally between
0.5 and 1.5 pounds per horsepower per pound (1.0 - 2.0 lb/O2/hp-hr).
High efficiency air stones which can create finer bubbles have higher
SAE closer to 3.0 pounds of oxygen per horsepower per hour. Understanding
the needs of your aquatic system will help you best choose the diffuser
system that will be most cost-effective for your project. Depth of
the diffuser placement will dictate the PSI (Pounds per Square Inch)
requirements of your compressor or blower. Compressors and blowers
have limited airflow and pressure thresholds and tubing lengths can
further impact the final diffuser performance. In deeper ponds and
lakes over 10 foot depth the best aeration efficiency and oxygen transfer
will be with a bottom mount diffuser system. Rotary vane or piston
compressors can provide adequate airflow in the most common situations
and if higher airflow and pressure is necessary then a rotary screw
compressor. It is important to recognize and understand the differences
between reciprocating, carbon vane, rotary centrifugal, regenerative
blower and rotary screw compressors as they all have specific strengths
and weaknesses.

To maximize the efficiency of a diffuser system and to get the most
pounds of oxygen per horsepower per hour from the diffuser you need
to ensure the air blower system is tailored to your specific job.

One interesting benefit of using land-based blowers or air compressor
systems for pond aeration is the reduced underwater noise levels of
diffuser systems when compared to surface aerators. More and more
research is focused on understanding the impact of extraneous noise
on aquatic creatures. Measurements conducted under water are raising
red flags about the noise produced by devices such as paddle wheels,
agitators, airlifts and drilled pipe homemade DIY diffusers or spargers.
In non-living systems (cement basins or oil field tailing ponds) this
extraneous noise is not an issue.

Another benefit of bottom-mount diffusers, because the air supply
blower or compressor is onshore and often hundreds of feet from the
diffuser, is that there is much less audible noise in the water column
to disrupt fish or aquatic life. Reducing the noise that is pushed
into the pond through an aeration device can improve the health of
the fish stocks and general well-being of the pond. Some pond and
lake owners, especially fish clubs or conservation groups have reported
that diffused air is the only aeration method they can use that will
not disrupt breeding. The situations and considerations are virtually
endless, so take some time to research the best aeration method for
your particular application beyond looking at the Oxygen Transfer.

Pumping
Rates of Diffuser Aeration Systems
One of the most popular ways of comparing diffusers is to use the
pumping rate of the diffuser based on the CFM airflow (cubic feet
per minute) through the diffuser at different depths. Diffusers will
circulate more gallons per minute as the depth of the diffuser placement
increases even with the same CFM flow. By measuring the Gallons Per
Minute of pumping it is possible to establish how many diffusers are
required and the CFM needed to circulate a given volume of water.

Establishing concrete results is not easy. Testing of diffusers is
often paid for by the manufacturer of the system and the data released
are usually the results that show the product in the best possible
light featuring the highest oxygen transfer rates and the best aeration
efficiency

Gallons
Per Minute Pumping Rate
Based on listed CFM airflow per diffuser and depth of diffuser

Increasing the CFM will increase the pumping capacity of the diffuser
but every diffuser type has limitations and backflow headloss equations
to consider. Wastewater and tank diffusers are not considered in this
chart as they are more specialized. This chart should be used as a
general guide only without scientific acknowledgment or reference.
Data is condensed estimations based on manufacturer claims.See
our selection of Aeration Diffusers | Air
Compressors

If a diffuser placed at 5 foot depth is rated to pump or circulate
500 gallons per minute with airflow of 2 CFM, the same diffuser at
a depth of 12 feet might have a pumping capacity of 1500 gallons per
minute with the same airflow of 2 CFM. This is why bottom mounted,
lake-bed diffusers are typically used in deeper situations.

If you need to aerate a large pond that has 2.3 million gallons (approximately
an acre sized pond average 7 feet deep) and you want to do a complete
turnover of the total pond volume every 24 hours, you need to look
at the pumping rates of your chosen diffuser at depth and determine
how many diffusers will do the job.

If each of your diffusers has a rated pumping capacity of 1000 Gallons
Per minute with 2 CFM (Cubic Feet per Minute) of airflow then that
single diffuser would theoretically be able to do a complete pond
turnover in around 38.5 hours. (2,300,000 gallons divided by 1000
gallons per minute = 2300 minutes = 38.3 hours to pump 2.3 million
gallons).

If you used two diffusers for a total pumping capacity of 2000 gallons per
minute you would be able to do a full volume mix of the pond in 1150 minutes
or 19.2 hours. If you used three diffusers for a total pumping rate of 3000
gallons per minute you could do the complete pond turnover in 767 minutes
or 12.8 hours. So with three diffusers and 6 CFM you can pump the full volume
of an acre pond in close to 12 hours.

Keep in mind that the depth and shape of your pond will have an impact
on total pumping rates and just how thorough the mixing of the pond
is; stagnant pools can exist in ponds where the water won't be moved
unless correct placement of the diffuser takes these dead zones into
account.

Smaller
bubbles are better bubbles
Let's have a brief lesson in fine bubbles, it will be fun (well...not
exactly) and you will get a short course in why smaller is sometimes
better especially when it comes to diffuser bubbles!

Lesson 1 - Let's take a fairly large bubble as far as aeration
goes. Our large galloping bubble that is 0.8" in diameter, just
over 3/4 of an inch, or 20 mm has a full volume of 1.64 cubic inches
or 4.19 cubic centimeters. This same glorious orb has a surface area
of 5 inches or 12.6 square centimeters and that is what we can define
as a coarse bubble!

Lesson 2 - Now a coarse diffuser like an airstone would create
large bubbles like in lesson one but if we could use the same volume
of air but break that large sphere into smaller ones we could greatly
increase the available surface area which in turn enhances and embellishes
the aeration transfer capacity of the same air volume! If we replaced
that one large 3/4" wide bubble with tiny bubbles or micro-bubbles
that were 1/16 of an inch or 3 mm in diameter we could fit 296 of
these fine bubbles into the larger one of lesson 1! The combined surface
area of those 296 bubbles works out to over 33 square inches or 84
square centimeters! This is close to 7 times the total surface area...and
the surface is where the oxygen transfer occurs.

Lesson 3 - Now since the surface area is close to 7 times greater
we can theoretically aerate 7 times as much water with the same compressor
or blower or air pump depending on whether we have a fine bubble diffuser
or a coarse bubbler. Same air pump, totally different efficiency!!

Surface
Aerators
Surface aerators usually consist of a submersed motor suspended from
a float that drives an angled prop to create a frothing oxygen transfer
boil on the surface. In smaller ponds and aquaculture tanks these
are often retail units between ½ HP and 5 HP. Larger models up to
300 HP are used by commercial applications and wastewater treatment.
A basic calculation for oxygen transfer requirements or inadequate
surface aerator is to allow at least ½ HP of an efficient surface
aerator for every million gallons of pond volume. This is only a guide
used in smaller ponds. High density fish ponds or ponds with a higher
BOD should calculate at least ¾ HP per million gallons.

The most efficient small-scale surface aerators will have an oxygen
transfer rate based on standard aeration efficiency (SAE) of 3 pounds
per hour per horsepower. This oxygen transfer rate will depend on
motor efficiency and important factors like pond depth and temperature.
As we see in the chart below, a manufacturer's listed oxygen transfer
rate of 3 pound per hour per horsepower will depend on the starting
oxygen rate (either PPM or mg/litre) of the water. If this surface
aerator is put into a pond that has an existing Dissolved Oxygen (D.O)
rate of 5 PPM or 5 milligrams per litre and the temperature of the
water is 59 degrees F (15 degrees Celsius) the actual oxygen transfer
rate of the quoted 3 pounds per HP per hour aerator will only be 46%
of the listed efficiency or 1.38 pounds of oxygen per hour per horsepower
(HP).

The pumping rates of surface aerators and fountains are usually somewhat
lower than bottom diffuser systems. A typical medium sized surface
aerator from 1 to 7.5 HP which uses a propeller system and a float
will circulate anywhere from 200 to 3000 gallons per minute. The energy
required to run a 7.5 HP surface aerator to move 3000 gallons per
minute would be more than would be required for a diffuser system
which might be able to move the same volume of water with a 1/3 HP
air pump and three diffusers.

A surface aerator would be a poor choice for a large or deep pond.
It will continue to pump basically the same water over and over again,
adding no oxygen where it is needed, because it is not moving water
away from the aerator. Paddlewheels or aspirator type surface systems
would be better suited when the movement of the oxygenated water away
from the aeration device is important.

For the smaller scale surface aerator we use Kasco Marine surface
aerators or Aquamaster Surface Aerators or Scott Aerators or any of
the many popular retail brands.The typical application include commercial
aquaculture, agricultural ponds, industrial plants, municipal waste
water and backyard ponds. These units are typically selected for surface
aeration when a decorative fountain pattern is neither necessary nor
desired, many pond aerators are self-contained, lightweight units
that float at the surface with a single power cord returning to shore
and two or three mooring lines anchoring the unit. The units are easily
installed and maintained by a single person and have proven to be
an excellent choice for both continuous duty pond and lake aeration
and supplemental aeration for unique applications.

Paddle
Wheel Surface Aerators
Paddlewheel aerators are surface aerators consisting some sort of
motor, electric, diesel, gas powered or tractor power takeoff that
is attached to a floating structure and paddles attached to the motor
hub or shaft which spin and splash at the water surface to create
aeration. By increasing the diameter of the paddles the oxygen transfer
rate can be increased; other important variables are the specific
depth of the blades as well of the angle of the paddles. By increasing
motor speed and hub rotation the oxygen transfer rate is also incrementally
enhanced.

A tractor powered takeoff (PTO) can have very high SOTR (Standard
Oxygen Transfer Rate) as much as 90 pounds of oxygen per hour which
means they can be important as an emergency aeration supply but the
drive-train and drive-shaft of such systems drain efficiency and they
are not at all energy efficient beyond emergency aeration. The more
efficient three-phase or 3-phase power paddle wheel aerators have
a SAE of between 3.5 and 6 pounds of oxygen per HP per hour. This
high SAE, combined their ability to de-stratify through rapid circulation
makes this type of aerator very popular with aquaculture around the
world.

Aspiration
and Jet Aerators
This is another surface mounted aeration device that is very good
at adding aeration and circulation to a pond or basin. They use a
submersed propeller that creates a vacuum effect and draws air through
intake ports into a hollow shaft that is dispersed in a large plume
of fine bubbles (about 2 mm in diameter) throughout the water. The
angle of attack can be adjusted with simple altering of the float
brackets. The aspiration propeller aerators are extremely quiet as
there is no water boil at the surface like a standard surface aerator;
all of the mixing is below the surface. Motors can range from 1 to
100 HP and above. Multiple units can be deployed in basins to ensure
there are no stagnant dead spots. This creates a flow linkage that
disperses and mixes the oxygen evenly through the pond and ensures
an effective circulation. The SAE is approximately 2.0 - 3.0 LB/O2/hp-hr.

Dissolved
Oxygen Meters
Using a dissolved oxygen meter is the most accurate way to ensure
your pond is getting the oxygen it requires to remain healthy.

Oakton
Dissolved Oxygen 110 Meter, with Probe
Independent 100% and zero adjustment calibrations, and
auto-calculation of offset values for accuracy Automatic
temperature compensation (ATC) for ease of use Stores
and recalls up 100 dissolved oxygen (DO) readings with
corresponding temperature for reliability during field
use Water-resistant keypad for meter protection Dual-line,
LCD display of mg/L (ppm) or % saturation, plus temperature
in degrees C or F for flexibility of use

ThePondReport.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. Read our disclosure policy regarding the affiliations and commercial incentivizations of The Pond Report web site.